Synchronization method and apparatus for device to device communication in wireless communication system

ABSTRACT

Disclosed is a method of transmitting/receiving a signal by a transmission device in a mobile communication system. The method includes: receiving information related to synchronization signals from a Base Station (BS); transmitting a first synchronization signal based on the received information; and receiving a second synchronization signal corresponding to the first synchronization signal based on the received information. When a UE cannot receive a synchronization signal from the BS or is located beyond a coverage of the BS, or when the BS cannot operate due to an emergency or disaster situation, UEs can configure self synchronization therebetween in a predetermined area without any assistance from the BS and thus broadcast and unicast transmission are possible between the UEs based on the synchronization.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/905,773 filed on Jan. 15, 2016, which is a 371 ofInternational Patent Application No. PCT/KR2014/006389 filed on Jul. 15,2014, which claims priority to Korean Patent Application No.10-2013-0082896 filed on Jul. 15, 2013, the disclosures of which areherein incorporated by reference in their entirety.

BACKGROUND 1. Field

The present invention relates generally to a method and an apparatus forperforming direct communication between wireless devices in a wirelesscommunication system, and more particular to a method in which UEsbeyond a coverage of a BS obtain synchronization therebetween even in astate where there is no coverage of the BS and performtransmission/reception therebetween based on the obtainedsynchronization.

Further, the present invention relates to a method of obtaining selfsynchronization between UEs in an emergency and disaster situation andsupporting broadcast transmission or unicast transmission based on thesynchronization.

2. Description of Related Art

In general, mobile communication systems have been developed for apurpose of providing communication while securing mobility of a user.The mobile communication systems have reached a stage where a high speeddata communication service can be provided as well as voicecommunication on the strength of the rapid development of technologies.

Currently, a standardization operation from a 3rd Generation PartnershipProject (3GPP) system to a Long Term Evolution (LTE) system is beingprogressed as one of next generation mobile communication systems. TheLTE-A system has been developed to continuously improve the performanceand implements a high speed packet-based communication having atransmission rate 3 to 10 times higher than a currently provided datatransmission rate.

Further, a device to device communication has been continuouslyresearched since the 2000's. A current mobile communication system canperform communication such as data transmission/reception through awireless network of a Base Station (BS) and a wired network connected tothe network for data transmission between User Equipments (UEs) withinthe same BS or neighboring BSs. Accordingly, the device to devicecommunication has been researched to reduce loads and support effectivecommunication between neighboring UEs when there is no wireless network.In addition, device to device communication which can be effectivelyperformed in a state where there is no BS or the BS does not operate dueto emergency or disaster is being researched.

Hereinafter, it must be understood that the LTE system refers to ameaning including both the LTE system and the LTE-A system.

In device to device communication, different User Equipment's (UEs) cancommunicate with each other without interference therebetween only whensynchronization is maintained based on a Base Station (BS) signal in astate where a BS exists in a wireless communication system. Accordingly,the communication is operated based on a BS synchronization signal. Whenthe BS does not exist, the BS signal cannot be detected, or the BS doesnot operate due to an emergency or disaster situation, the BSsynchronization signal is not received by the UE, so that thecommunication is not possible. Therefore, in order to effectivelycommunicate in such a situation, a method of obtaining synchronizationby itself and a method and an apparatus for performing datacommunication between UEs based on the method are required.

SUMMARY

In accordance with an aspect of the present invention, a method oftransmitting/receiving a signal by a transmission device in a mobilecommunication system is provided. The method includes: receivinginformation related to synchronization signals from a Base Station (BS);transmitting a first synchronization signal based on the receivedinformation; and receiving a second synchronization signal correspondingto the first synchronization signal based on the received information.

In accordance with another aspect of the present invention, a method oftransmitting/receiving a signal by a reception device in a mobilecommunication system is provided. The method includes: receiving a firstsynchronization signal from a transmission device; and transmitting asecond synchronization signal corresponding to the first synchronizationsignal based on the first synchronization signal.

In accordance with another aspect of the present invention, atransmission device transmitting/receiving a signal in a mobilecommunication system is provided. The transmission device includes: atransceiver for transmitting/receiving a signal to/from at least one ofa Base Station (BS) and a reception device; and a controller forcontrolling the transceiver, receiving information related tosynchronization signals from the BS, transmitting a firstsynchronization signal based on the received information, and receivinga second synchronization signal corresponding to the firstsynchronization signal based on the received information.

In accordance with another aspect of the present invention, a receptiondevice transmitting/receiving a signal in a mobile communication systemis provided. The reception device includes: a transceiver fortransmitting/receiving a signal to/from at least one of a transmissiondevice and a Base Station (BS); and a controller for controlling thetransceiver, receiving a first synchronization signal from thetransmission device, and transmitting a second synchronization signalcorresponding to the first synchronization signal based on the firstsynchronization signal.

According to an embodiment of the present invention, when a UE cannotreceive a synchronization signal from the BS or is located beyond acoverage of the BS, or when the BS cannot operate due to an emergency ordisaster situation, UEs can configure self synchronization therebetweenin a predetermined area without any assistance from the BS and thusbroadcast and unicast transmission are possible between the UEs based onthe synchronization. Advantages, and salient features of the inventionwill become apparent to those skilled in the art from the followingdetailed description, which, taken in conjunction with the annexeddrawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionin conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a communication system between UEs to which anembodiment of the present invention is applied;

FIG. 2 illustrates a configuration of a downlink subframe of an LTEsystem to which an embodiment of the present invention is applied;

FIG. 3 illustrates a configuration of an uplink subframe of an LTEsystem to which an embodiment of the present invention is applied;

FIG. 4 illustrates a communication method between UEs provided by anembodiment of the present invention;

FIG. 5 illustrates a process of transmitting and receiving asynchronization signal for device to device communication provided by anembodiment of the present invention;

FIG. 6 illustrates a process of transmitting and receiving a datachannel for device to device communication provided by an embodiment ofthe present invention;

FIG. 7 illustrates a process of transmitting and receiving a datachannel for device to device communication provided by an embodiment ofthe present invention;

FIG. 8 illustrates operations of a UE for a synchronization methodprovided by an embodiment of the present invention;

FIG. 9 illustrates operations of a UE for data channel transmissionprovided by an embodiment of the present invention;

FIG. 10 illustrates operations of a UE for a synchronization methodprovided by an embodiment of the present invention;

FIG. 11 illustrates operations of a UE for data channel transmissionprovided by an embodiment of the present invention; and

FIG. 12 illustrates components of a UE according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

In describing the embodiments of the present disclosure, descriptions oftechnical details that are well-known in the art and are not directlyassociated with the embodiments of the present disclosure will beomitted. This is intended to prevent obscuring of the main idea of thepresent invention and more clearly transfer the main idea by omittingunnecessary descriptions.

In the following description of embodiments of the present disclosure, adetailed description of known functions or configurations incorporatedherein will be omitted when it is determined that the detaileddescription thereof may unnecessarily obscure the subject matter of thepresent invention. Hereinafter, embodiments of the present disclosurewill be described with reference to the accompanying drawings.

Embodiments of the present invention are for device to devicecommunication performed by a UE using downlink or uplink in acommunication system in which a BS transmits a downlink signal to the UEand the UE transmits an uplink signal to the BS, the UE being able toaccess an LTE system. An LTE downlink signal includes a data channelincluding information, a control channel transmitting a control signal,and a Reference Signal (RS) for channel measurement and channelfeedback. An LTE uplink signal includes a data channel includinginformation, a control channel transmitting feedback information or acontrol signal, and a Sounding Reference Signal (SRS) for measuring achannel of the UE by the BS.

An LTE BS transmits data information and control information to the UEthrough a Physical Downlink Shared CHannel (PDSCH) and a DownLinkControl CHannel (DLCCH), respectively.

The uplink includes a data channel, a control channel, and a referencesignal which the UE transmits to the BS, and the data channel istransmitted through a Physical Uplink Shared CHannel (PUSCH) and thecontrol channel is transmitted through a Physical Uplink Control CHannel(PUCCH).

The LTE BS may have a plurality of reference signals, and the pluralityof reference signals may include one or more of a Common ReferenceSignal (CRS), a Channel State Information RS (CSI-RS), and aDeModulation Reference Signal (DMRS) or a UE-specific reference signal.

The CRS is transmitted over full downlink bandwidths and used when allUEs within a cell demodulate signals and measure channels. In order toreduce resources used for CRS transmission, the BS transmits aUE-specific DMRS only to an area scheduled for the UE with theUE-specific DMRS and transmits a CSI-RS on time and frequency axes toobtain channel information.

The UE transmits the data channel (PUSCH) and the control channel(PUCCH) by using the UE-specific DMRS and may also transmit a SoundingReference Signal (SRS) to measure an uplink channel. The SRS istransmitted to a last symbol of a subframe and cannot be transmittedsimultaneously with the PUSCH and the PUCCH. In general, when the PUCCHis transmitted at an edge of the uplink bandwidth, the PUSCH may betransmitted over the full bandwidths.

FIG. 1 illustrates a system for device to device communication in acommunication system in which the BS transmits/receives data to/from theUE.

Referring to FIG. 1, BSs 103 and 105 may communicate with UEs 108 and109 and such communication may be supported within a coverage of the BSwhere a synchronization signal of the BS is transmitted.

At this time, data to be transmitted to the UEs 108 and 109 istransmitted from a network 101 to the BS 105, and the BS 105 schedulesradio resources and transmits data 115 to the UE 108.

However, when the BS 103 loses the coverage due to an emergency ordisaster situation, the UE 109 cannot communicate any more since the UE109 cannot find the BS. In this case, when the device to devicecommunication supports communication between the UEs 109 which are notincluded within the coverage of the BS and thus the communicationbetween the UEs 109 is performed, data is transmitted and users usingthe UEs can communicate with each other.

FIG. 2 illustrates a downlink subframe.

Referring to FIG. 2, a scheduling unit of the BS is a downlink subframe201 or 203, and one subframe 201 or 203 includes two slots 205corresponding to a total of N_(symb) ^(DL) symbols and transmits acontrol channel, a data channel, and a reference signal.

Among the N_(symb) ^(DL) symbols, chronologically earlier M_(symb) ^(DL)symbols are used for transmitting a control channel 211 and theremaining symbols N_(symb) ^(DL)−M_(symb) ^(DL) are used fortransmitting a data channel 213.

A transmission bandwidth is configured by Resource Blocks (RBs) 217 onthe frequency. Each of the RBs 217 consists of a total of N_(SC) ^(RB)subcarriers or Resource Elements (REs) and a unit including two slots onthe time axis and one RB is referred to as a PRB pair. The PRB pairtransmits a Common Reference Signal (CRS) 209, A Channel StateInformation-Reference Signal (CSI-RS), and a DeModulation RS (DMRS) 207.

In order to measure a downlink channel, the BS transmits the CRS or theCSI-RS to allow the UE to perform the channel measurement. When the UEreports a result of the channel measurement to the BS, the BS can know astatus of a downlink channel between the BS and the UE.

A downlink subframe may be divided into a normal subframe 201 and anMBSFN subframe 203 according to a structural characteristic thereof.Equally to the normal subframe, the MBSFN subframe performs transmissionin the first two symbols as indicated by a reference numeral 215 butdoes not transmit the CRS in the remaining symbols which is a differencebetween them.

FIG. 3 illustrates a configuration of an uplink subframe of the LTEsystem to which an embodiment of the present invention is applied.

Referring to FIG. 3, the BS instructs the UE to transmit an SRS 309 soas to perform uplink channel measurement. Specifically, the BS canrecognize the uplink channel (channel status) by receiving the SRS 309transmitted by the UE. The UE uses an uplink subframe 301 as a basictime unit for the transmission and each of uplink subframes 301 and 302includes two slots. The subframe consists of a total of N_(symb) ^(UL)symbols, and the UE transmits a control channel, a data channel 307, areference signal 305 and the like through the symbols.

The control channel (PUCCH) is transmitted at an edge on the frequencyin the uplink band and one PUCCH may be transmitted alternately at bothuplink edges in the unit of slots.

The control channel and the data channel allocate parts of the symbolsof an allocation region to the RS (DMRS) 305 as illustrated in FIG. 1,so that the BS can demodulate the signal transmitted by the UE. The SRSis transmitted to the last symbol of the subframe 302 as indicated by areference numeral 309. At this time, the data channel is not transmittedto the last symbol where the SRS is transmitted.

The device to device communication may support largely two types ofcommunication. The first type of communication corresponds to unicasttransmission and refers to transmission by direct communication betweenone UE and another UE. The second type of communication corresponds tomulticast or broadcast transmission and refers to transmission in whichone UE transmits the same data to an unspecified multitude of UEs. Theunicast transmission refers to data transmission such as voice datatransmission or a messenger to a specific user, and the multicast orbroadcast transmission may include communication such as emergencyinformation, advertisement, marketing, or traffic informationtransmitted to unspecified multitudes.

When the UE cannot identify the BS any more since the UE escapes from anarea of the BS for a long time or the UE is located at a coverage hole,or when the BS is damaged or cannot operate since an emergency ordisaster situation such as blackout, earthquake, tidal wave, typhoon,hurricane, or tsunami occur, the UE cannot communicate with the BS andUEs lose all synchronizations. In such a disaster situation, functionsof the UE are not damaged. Accordingly, if communication between UEs ispossible, adjacent UEs can transmit/receive data, so that an emergencyrescue team may arrive and recognize a signal for help to save a life.Further, when a predetermined BS cannot operate, communication betweenUEs connected to the corresponding BS is possible since the UEs arelocated physically close to each other. However, when the BS cannotoperate, the UEs having received communication from the corresponding BSlose synchronization and the UEs having lost the synchronization cannotperform communication even though they actually have a UE communicationfunction.

As described above, it is required to secure the synchronization betweenUEs in order to transmit/receive data between the UEs which go beyondthe coverage of the BS or cannot transmit/receive data to/from the BS.When the BS cannot communicate, a subject such as the BS which startssynchronization for UEs or provides information on the synchronizationdisappears. Accordingly, the UEs are required to obtain synchronizationthrough their own determination by themselves and thus a separatesynchronization structure different from the LTE system for datatransmission/reception to/from the BS is needed. The separatesynchronization structure corresponds to a method of effectivelyobtaining synchronization and obtaining synchronization by itselfwithout separate signaling.

FIG. 4 illustrates a synchronization transmission method provided by anembodiment of the present invention.

In the embodiment, the provided synchronization transmission method usesa first synchronization signal and a second synchronization signal. TheUE searches for the first synchronization signal or the secondsynchronization signal in an asynchronous state. When both the first andsecond synchronization signals are not found in the asynchronous state,the UE transmits the first synchronization signal. When the firstsynchronization signal is found but the second synchronization signal isnot found, the UE synchronizes itself with the first synchronizationsignal and transmits only the second synchronization. When the secondsynchronization signal is found but the first synchronization signal isnot found, the UE synchronizes itself with the second synchronizationsignal and transmits only the first synchronization signal. When boththe first and second synchronization signals are found, the UEsynchronizes itself with the first and second synchronization signalsand transmits no synchronization signal. In some embodiments, the firstsynchronization signal and the second synchronization signal may besynchronization signals having preset different sequences. Further, inanother embodiment, the first synchronization signal and the secondsynchronization signal may be synchronization signals transmitted orreceived according to different periods.

Referring to FIG. 4, a first user 401 corresponds to a UE of which firstsynchronization and second synchronization are not detected and maydetermine whether the first synchronization and the secondsynchronization exist for a predetermined time. When the firstsynchronization signal and the second synchronization signal are notdetected or when the first synchronization signal and the secondsynchronization signal do not have a high enough signal quality, the UEof the first user 401 transmits the first synchronization signal withina range including an area indicated by a reference numeral 411 near theUE. The range within which the first synchronization signal istransmitted may be selectively determined according to the UE. In thiscase, the area of the first synchronization signal is formed near the UEaccording to transmission power of the first synchronization signaltransmitted by the first user 401 and a channel status, as indicated bythe reference numeral 411. At this time, UEs corresponding to a seconduser 403 receive the first synchronization signal transmitted by thefirst user 401. However, since the first user 401 does not transmit thesecond synchronization signal, the UEs corresponding to the second user403 receive only the first synchronization signal and the second usertransmits the second synchronization signal. When the UEs correspondingto the second user 403 transmit the second synchronization signal, asecond synchronization signal area 413 may be formed near the UEs of thesecond user 403 while partially overlapping the first synchronizationsignal area. When a third user 405 can receive only the secondsynchronization signal even though the UEs corresponding to the firstuser 401 and the second user 403 transmit the first synchronizationsignal and the second synchronization signal, the third user 405transmits only the first synchronization signal. When a fourth user 407can receive both the first synchronization signal and the secondsynchronization signal, the fourth user 407 transmits nothing. In someembodiments, the synchronization signal transmission method may includea step of, when both synchronization signals are received, selecting asynchronization signal having reception quality lower than presetreception quality between the two received signals and re-transmittingthe selected synchronization signal.

In a provided embodiment, determining whether the synchronizationsignals are received may include a case where reception quality of thesynchronization signal is larger than or equal to a particular referenceand is configured in advance by signaling, a case where the receptionquality is written in a memory of the UE in advance, or a case wherereception power is determined to be larger than or equal to apredetermined reference. In the provided embodiment, determining thetransmission power of the synchronization signal includes all of a casewhere the synchronization signal is transmitted in inverse proportion tothe received synchronization signal quality or configured by highersignaling, a case where the transmission power is written in a memory ofthe UE in advance, or a case where the same transmission power is alwaysused.

In the provided embodiment, an interval between the firstsynchronization signal and the second synchronization signal and aninterval between the second synchronization signal and the firstsynchronization signal are constant while a predetermined time internalbetween the first synchronization signal and the second synchronizationsignal is maintained. This is because the UE cannot know when thesynchronization signal is transmitted in an asynchronous state and alsois to include a time when a receiver and a transmitter are switched. Inthe provided embodiment, transmission signals of the firstsynchronization signal and the second synchronization signal aretransmitted using particular sequences, and the first synchronizationsignal and the second synchronization signal use different sequenceswhich can be distinguished from each other. However, an embodiment ofthe present invention includes a case where the first synchronizationsignal and the second synchronization signal use the same sequence. Whenthe first synchronization signal and the second synchronization signaluse the same sequence, the UE searches for the sequence on a period of 5msec by using one sequence. When the found sequence is received on aperiod of 10 msec, the UE recognizes that only one of the firstsynchronization signal and the second synchronization signal is detectedand transmits the other signal after 5 msec. Accordingly, thedistinguishment between the first synchronization signal and the secondsynchronization signal is made through not only the use of differentsequences but also the use of the same sequence transmitted at differenttime. Further, in some embodiments, the first synchronization signal andthe second synchronization signal have the same period and may havedifferent offset values. More specifically, the first synchronizationsignal and the second synchronization signal have a period of 10 msec,and the first synchronization signal has an offset value of 0 msec andthe second synchronization signal has an offset value of 5 msec. Throughsuch a configuration, the first synchronization signal and the secondsynchronization signal are distinguished and transmitted using onesequence. Information related to the above described synchronizationsignals may be received from the BS or may be preset in the UE.

FIG. 5 illustrates a process of transmitting/receiving, on a time axis,the first synchronization signal and the second synchronization signalprovided by an embodiment of the present invention.

Referring to FIG. 5, a first user 510, a second user 520, a third user530, and a fourth user 540 are users which may correspond to the firstuser 401, the second user 403, the third user 405, and the fourth user407 of FIG. 4.

The first synchronization signal and the second synchronization signalare transmitted in radio frames 501 and 502 of the LTE system and theradio frame 501 having a length of 10 msec is illustrated for anexemplary embodiment. However, the signals may be used for a framehaving a different length. FIG. 5 illustrates a synchronization signaltransmitting/receiving operation according to an embodiment of thepresent invention from a viewpoint of a transmitter and a receiver ofeach user.

The first user 510 is in an asynchronous state and has received nosynchronization signal. In this case, the first user 510 operates areceiver in a reception mode in the radio frame 501 to attempt toreceive the first synchronization signal and the second synchronizationsignal. The embodiment of the present invention includes a case whereone or more radio frames 501 are used for reception. When the first user501 has received no synchronization signal, the first user 501 turns offa receiver 511 in a part 513 of the radio frame 502 and transmits afirst synchronization signal 515 to a transmitter 512. However, in someembodiments, the first user 510 may operate the transmitter 512 withoutturning off the receiver 511.

When the second user 520 receives a first synchronization signal 523 ina state where a receiver 521 is in a reception mode, the second user 520continuously maintains first synchronization signals 523 and 525 in thereception state in a radio frame 504 and then transmits the secondsynchronization signal at a time point 527 when the secondsynchronization signal should be transmitted.

The third user 530 receives only a second synchronization signal 533 ina state where a receiver 531 is in a reception mode in a radio frame505. In this case, the third user 530 transmits a first synchronizationsignal 535 while continuously receiving a second synchronization signal537 in a radio frame 506. The fourth user 540 is a user who receivesboth a first synchronization signal 543 and a second synchronizationsignal 545. The fourth user 540 mostly receives the same firstsynchronization signal 543 and second synchronization signal 545 from aplurality of users, but the reception may have a little error accordingto a distance of the UE performing the transmission. However, sinceseveral UEs perform the transmission, reception quality can be improved.

The fourth user 540 transmits nothing when both the firstsynchronization signal 543 and the second synchronization signal 545have high reception quality as described above. However, when the signalquality of the second synchronization signal 545 is lower than areference, the second synchronization signal can be transmitted asindicated by a reference numeral 549.

As illustrated in FIGS. 4 and 5, when each of the users determines thesynchronization signal transmitted by itself according to the number ora period of received synchronization signals, an area formed using thefirst synchronization signal, an area formed using the secondsynchronization signal, and an area formed using both the firstsynchronization signal and the second synchronization signal exist inparticular regions between the UEs and there are UEs synchronized usingthe first synchronization signal and UEs synchronized using the secondsynchronization signal. Since the UE synchronized using the firstsynchronization signal transmits the second synchronization signal andthe UE synchronized using the second synchronization signal transmitsthe first synchronization signal, the UE synchronized using the firstsynchronization signal and the UE synchronized using the secondsynchronization signal may have the same synchronization therebetween.

After the synchronization of the UEs is formed, broadcast or unicasttransmission can be performed between the UEs. In general, the broadcasttransmission is needed most in a disaster situation, and the unicasttransmission may be performed in the same way. That is, in an embodimentof the present invention, the UE receives all data channels. However, inthe unicast transmission, the UE performs only a reception operationwithout a transmission operation for the received data channel. In thebroadcast transmission, the UE performs both the reception operation andthe transmission operation. Further, in the unicast transmission, the UEinserts identification information such as a UE ID into the transmitteddata and transmits the data to one or more of specified transmission andreception UEs, so as to perform the unicast transmission.

FIG. 6 illustrates a data channel transmission method after thesynchronization provided by an embodiment of the present invention.

Referring to FIG. 6, a first user 610, a second user 620, a third user630, and a fourth user 640 are users corresponding to the first user510, the second user 520, the third user 530, and the fourth user 540 ofFIG. 5, respectively.

The first user 610 continuously transmits a first synchronization signal613 to a UE which has initially configured the synchronization in aradio frame 601 to configure the network synchronization. At this time,the UE transmits a data channel 617 at a predetermined time point aftertransmitting a first synchronization signal 615. In an exemplaryembodiment, it is assumed that the transmitted data is broadcast datawhich all UEs receive. In this case, after maintaining thesynchronization while receiving a first synchronization signal 623, thesecond user 620 continuously waits for the data channel after the firstsynchronization signal. When the data channel 617 is transmitted to thefirst user 610, the second user 620 recognizes generation of a datachannel 625. The second user 620 demodulates the data channel 625 andre-transmits the data channel 625 after a predetermined time 629 from asecond synchronization signal 628 transmitted by the second user 620.The re-transmitted data channel 629 is the same as the received datachannel 625. UEs synchronized using a second synchronization signal 634such as the third user 630 receive a data channel 635 re-transmitted bythe second user 620. In this case, like the second user 620, the thirduser 630 re-transmit a data channel 637 after transmitting a firstsynchronization signal 636. A UE such as the fourth user 640 receivingboth a first synchronization signal 643 and a second synchronizationsignal 647 may not perform the transmission any more.

In such a data transmission method, after data channel transmissionfirst starts, other UEs continuously re-transmit data. In this case, thetransmitted data continuously overlaps. Accordingly, only when the UEinitially performing transmission repeatedly transmits the same data fora predetermined transmission time, the data transmission quality isimproved in the entire network and the data is propagated to UEsperforming re-transmission. Further, only when the UE stops transmittinginitially transmitted data channels and waits for a predetermined time,the UEs performing the re-transmission stops the re-transmission. Such aprocess is referred to as a transmission period and a refresh period,and FIG. 7 illustrates the transmission process.

FIG. 7 illustrates a process of transmitting and receiving a datachannel for device to device communication provided by an embodiment ofthe present invention.

Referring to FIG. 7, when a first user 710 transmitting a first datachannel 709 transmits the data channel 709, the first user 710 maytransmit the same data channel by using a transmission period 701 untila second user 720, a third user 730, and a fourth user 740 re-transmitthe same data channel to a large enough coverage. In some embodiments,whether the data channel is re-transmitted to the large enough coveragemay be determined based on one or more of a preset time period, alocation relation between adjacent UEs, and reception quality of thedata channel which the UE receives.

Further, a refresh period 703 is configured before the first user 710transmits a next data channel 711 and thus re-transmitted data is faraway from the place where the user first transmitted the data, so thattransmission of the new data 711 does not collide with a previoustransmission. When the refresh period 703 is not made long enough, thesecond data channel 711 may collide in a side of a receiver, andaccordingly broadcast transmission is not possible.

In unicast transmission, the data channel is transmitted in the same wayas described above. When the user demodulates the data channel and adestination corresponds to the user, the user does not performre-transmission. When the destination does not correspond to the user,the user performs the re-transmission. Determination of whether thedestination is the user may be performed based on a destinationindicator included in the data channel in some embodiments.

In an embodiment of the present invention, one or more different datachannels may exist simultaneously, chronologically, or frequencydivisionally and a plurality of users perform transmission in one radioframe.

In the embodiment of the present invention, although it has been assumedand described that the UE is in the asynchronous state where there is noBS, only the first user can perform transmission/reception with the BSand the remaining users cannot perform transmission/reception with theBS or the first user operates as a temporary BS in an embodiment of thepresent invention.

Further, in some embodiments, the UE having successfully received thedata channel may control power allocation of the data channel which theUE will transmit based on reception channel of the received datachannel. As described above, it is possible to prevent the same datachannel from being transmitted to the UE which has transmitted the datachannel by controlling power allocation of the data channel. Morespecifically, the UE in a transmission side may insert allocated powerinformation into the data channel when the data channel is transmitted.

FIG. 8 is a flowchart illustrating a synchronization operation of the UEwhen all users are in an asynchronous state according to an embodimentof the present invention.

Referring to FIG. 8, in step 801, the UE may attempt to receive thefirst synchronization signal and the second synchronization signal.

In step 803, the UE may determine whether the first synchronizationsignal has been successfully received.

As a result of the determination of step 803, when the terminal hasfailed to receive the first synchronization signal, the UE may determinewhether the second synchronization signal has been successfully receivedin step 805.

As a result of the determination of step 805, when the UE has failed toreceive the second synchronization signal, the UE transmits the firstsynchronization signal in step 807.

As a result of the determination of step 805, when the UE hassuccessfully received the second synchronization signal, the UE maytransmit the first synchronization signal in step 809.

As a result of the determination of step 803, when the terminal hassuccessfully received the first synchronization signal, the UE maydetermine whether the second synchronization signal has beensuccessfully received in step 811.

As a result of the determination of step 811, when the UE has failed toreceive the second synchronization signal, the UE may transmit thesecond synchronization signal in step 813.

As a result of the determination of step 811, when the UE hassuccessfully received the second synchronization signal, the UE haseventually received both the first synchronization signal and the secondsynchronization signal, and the UE may transmit any of the firstsynchronization signal and the second synchronization signal which haslower reception capability and does not reach a preset receptioncapability reference in step 815. In some embodiments, the presetreception capability reference may be determined based on an SINR value.Further, the UE may transmit one of the first synchronization signal andthe second synchronization signal which has lower reception quality.

FIG. 9 is a flowchart illustrating a data channel transmitting operationof the UE when all users are in a synchronous state according to anembodiment of the present invention.

Referring to FIG. 9, operations in steps 901 to 915 may correspond tosteps 801 to 815 of FIG. 8, respectively.

In step 917, when the UE has successfully received the synchronizationsignals and has successfully modulated the data channel, the UE maytransmit the same data channel as the data channel received at apredetermined time after the synchronization signal transmitted by theUE itself.

In some embodiments, synchronization signals including the firstsynchronization signal and the second synchronization signal and datachannels may be transmitted in different channels. More specifically,the synchronization signals are transmitted in a downlink channel andthe data channels are transmitted in an uplink channel.

In another embodiment, the synchronization signal and the data signalmay be transmitted in the same channel.

FIG. 10 illustrates an operation of the BS in which a particular userconfigures a network for an asynchronous UE by an instruction of the BSaccording to an embodiment of the present invention.

Referring to FIG. 10, in the BS instructs the UE to receive the firstsynchronization signal and the second synchronization signal.

In step 1001, the BS transmits configuration information of the firstsynchronization signal and the second synchronization signal to the UE.

In step 1003, the BS identifies whether the first and secondsynchronization signals are received from the UE. It is to determinewhether there is pre-configured synchronization transmission.

Thereafter, when the BS determines that the synchronization signal canbe transmitted, the BS instructs the UE to transmit the firstsynchronization signal or the second synchronization signal in step1005.

Next, in step 1007, the BS may instruct the UE to transmit data as wellas the synchronization signals, which may be performed similarly to step917 of FIG. 9.

FIG. 11 illustrates an operation of the UE in which a particular userconfigures a network for an asynchronous UE by an instruction of the BSaccording to an embodiment of the present invention.

Referring to FIG. 11, in step 1101, the UE receives configurationinformation of the first synchronization signal and the secondsynchronization signal from the BS.

M step 1103, the UE identifies whether the BS has received the first andsecond synchronization signals.

In step 1105, the UE receives an instruction to transmit thesynchronization signal from the BS.

In step 1107, the UE receives an instruction to transmit data from theBS and transmits a data channel together with the synchronizationsignal. In some embodiments, an operation of step 1107 may beselectively performed.

FIG. 12 is a block diagram illustrating an internal structure of a UEdevice supporting synchronization between UEs and data channelsynchronization provided by an embodiment of the present invention.

Referring to FIG. 12, a UE controller 1209 receives a synchronizationsignal through a synchronization receiver 1207 by controlling areception device 1201 and a transmission device 1203 through a timeduplexer 1205 and transmits a synchronization signal through asynchronization signal generator 1213 according to the receivedsynchronization signal. Thereafter, the UE controller 1209 receives datathrough a data channel receiver 1211 based on a data channel transmittedafter the synchronization signal and re-transmits the data by using adata channel transmitter 1215. Further, the control of the operationsmay be performed through a controller (the controller 1209).

Embodiments of the present invention disclosed in the specification andthe drawings are only particular examples to easily describe thetechnical matters of the present invention and assist in understandingof the present invention, and do not limit the scope of the presentinvention. It will be apparent to those having ordinary knowledge in thetechnical field, to which the present disclosure pertains, that it ispossible to practice other modified embodiments based on the technicalidea of the present disclosure as well as the embodiments disclosedherein.

Further, a method and an apparatus for device to device communication ina wireless communication system according to an embodiment of thepresent invention includes, as operations of the UE, an operation forreceiving synchronization signal configuration information between UEsthrough higher signaling, an operation for searching for asynchronization signal in an asynchronous state, an operation fortransmitting a synchronization signal in the asynchronous state, andmore specifically, an operation for searching for a firstsynchronization signal or a second synchronization signal, an operationfor transmitting the first synchronization signal when both the firstsynchronization signal and the second synchronization signal are notfound in the asynchronous state, an operation for searching for thefirst synchronization signal and for performing synchronization with thefirst synchronization signal and transmitting the second synchronizationsignal when the second synchronization signal is not found in theasynchronous state, an operation for searching for the secondsynchronization signal and for performing synchronization with thesecond synchronization signal and transmitting the first synchronizationsignal when the first synchronization signal is not found in theasynchronous state, an operation for performing synchronization with thefirst synchronization signal and the second synchronization signal andtransmitting no synchronization signal when both the firstsynchronization signal and the second synchronization signal are foundin the asynchronous state, an operation for determining whether totransmit the synchronization signal based on reception signal quality ofthe received synchronization signal when the synchronization signal isfound in the asynchronous state, and an operation for determiningwhether to transmit the synchronization signal based on reception signalquality of the received synchronization signal when the synchronizationsignal is found in the asynchronous state.

Operations after the synchronization between UEs is obtained provided byan embodiment of the present invention include an operation fortransmitting a data channel together with the first synchronizationsignal when only the first synchronization signal is transmitted in asynchronous state, an operation for transmitting only the secondsynchronization signal in the synchronous state, an operation forrepeatedly transmitting a data channel during a predetermined period inthe synchronous state, and an operation for transmitting no data channelfor a predetermined time after repeatedly transmitting the data channelduring the predetermined period.

In addition, an operation for performing synchronization with the firstsynchronization signal and the second synchronization signal andtransmitting no synchronization signal when both the firstsynchronization signal and the second synchronization signal are foundin the asynchronous state, an operation for determining whether totransmit the synchronization signal based on reception signal quality ofthe received synchronization signal when the synchronization signal isfound in the asynchronous state, and an operation for determiningwhether to transmit the synchronization signal based on reception signalquality of the received synchronization signal when the synchronizationsignal is found in the asynchronous state are included.

A UE device according to an embodiment of the present invention mayinclude a UE controller for recognizing a status and instructing toreceive and transmit a synchronization signal and a data channel, asynchronization signal generator for generating the first and secondsynchronization signals, a data channel transceiver for performing datatransmission/reception after the synchronization, and a synchronizationsignal receiver for recognizing the reception of the synchronizationsignal.

Further, an embodiment of the present invention provides asynchronization method and apparatus for device to device communicationin a wireless communication system, and more particularly to a method inwhich, when the UE is beyond the coverage of the BS or the BS does notoperate due to a disaster situation, UEs obtain synchronization bythemselves, configure a network therebetween in a particular area byusing the obtained synchronization, and effectively transmit broadcastor unicast data channels based on the configured synchronization.Particularly, the embodiment of the present invention includes a methodof determining whether to perform re-transmission according to a degreeof the obtained synchronization of the first and second synchronizationsignals by the UE in one radio frame and obtaining entiresynchronization.

Although exemplary embodiments of the present invention have been shownand described in this specification and the drawings, they are used ingeneral sense in order to easily explain technical contents of thepresent invention, and to help comprehension of the present invention,and are not intended to limit the scope of the present invention. It isobvious to those skilled in the art to which the present inventionpertains that other modified embodiments on the basis of the spirits ofthe present invention besides the embodiments disclosed herein can becarried out.

What is claimed is:
 1. A method performed by a first terminal operatingout of a coverage of a base station, the method comprising: obtainingconfiguration information including information on a first transmissiontiming and a second transmission timing associated with asynchronization signal, wherein the first transmission timing and thesecond transmission timing occur in one or more transmission periods;identifying whether a first synchronization signal is received from asecond terminal in one of the first transmission timing and the secondtransmission timing in at least one of the one or more transmissionperiods; in a case that the first synchronization signal is received inthe first transmission timing, transmitting a second synchronizationsignal in the second transmission timing; and in a case that the firstsynchronization signal is received in the second transmission timing,transmitting the second synchronization signal in the first transmissiontiming.
 2. The method of claim 1, wherein the first transmission timingand the second transmission timing are configured such that atransmission timing of the first synchronization signal is differentfrom a transmission timing of the second synchronization signal.
 3. Themethod of claim 1, further comprising: transmitting the secondsynchronization signal in the first transmission timing or the secondtransmission timing in a case that the first synchronization signal isnot received in the first transmission timing and the secondtransmission timing.
 4. The method of claim 1, wherein a sequence of thesecond synchronization signal is identified based on a sequence of thefirst synchronization signal.
 5. The method of claim 1, wherein theconfiguration information on the first transmission timing and thesecond transmission timing for a synchronization signal is preconfiguredor configured by a higher layer signaling.
 6. The method of claim 1,wherein receiving the first synchronization signal comprises:identifying a signal strength related to the first synchronizationsignal; and identifying, in a case that the signal strength is below athreshold value which is preconfigured or configured by a higher layersignaling, that the first synchronization signal is not received.
 7. Themethod of claim 1, wherein a power for transmitting the secondsynchronization signal is identified based on information which ispreconfigured or configured by a higher layer signaling.
 8. The methodof claim 1, wherein the configuration information on a firsttransmission timing and a second transmission timing for asynchronization signal includes a first offset for the firsttransmission timing and a second offset for the second transmissiontiming.
 9. A first terminal operating out of a coverage of a basestation, the first terminal comprising: a transceiver; and a controllercoupled with the transceiver and configured to: obtain configurationinformation including information on a first transmission timing and asecond transmission timing associated with a synchronization signal,wherein the first transmission timing and the second transmission timingoccur in one or more transmission periods, identify whether a firstsynchronization signal is received from a second terminal in one of thefirst transmission timing and the second transmission timing in at leastone of the one or more transmission periods, in a case that the firstsynchronization signal is received in the first transmission timing,transmit a second synchronization signal in the second transmissiontiming, and in a case that the first synchronization signal is receivedin the second transmission timing, transmit the second synchronizationsignal in the first transmission timing.
 10. The first terminal of claim9, wherein the first transmission timing and the second transmissiontiming are configured such that a transmission timing of the firstsynchronization signal is different from a transmission timing of thesecond synchronization signal.
 11. The first terminal of claim 9,wherein the controller is further configured to transmit the secondsynchronization signal in the first transmission timing or the secondtransmission timing in a case that the first synchronization signal isnot received in the first transmission timing and the secondtransmission timing.
 12. The first terminal of claim 9, wherein asequence of the second synchronization signal is identified based on asequence of the first synchronization signal.
 13. The first terminal ofclaim 9, wherein the configuration information on the first transmissiontiming and the second transmission timing for a synchronization signalis preconfigured or configured by a higher layer signaling.
 14. Thefirst terminal of claim 9, wherein the controller is further configuredto: identify a signal strength related to the first synchronizationsignal, and identify, in a case that the signal strength is below athreshold value which is preconfigured or configured by a higher layersignaling, that the first synchronization signal is not received. 15.The first terminal of claim 9, wherein a power for transmitting thesecond synchronization signal is identified based on information whichis preconfigured or configured by a higher layer signaling.
 16. Thefirst terminal of claim 9, wherein the configuration information on afirst transmission timing and a second transmission timing for asynchronization signal includes a first offset for the firsttransmission timing and a second offset for the second transmissiontiming.